It calls for radical change, saying Ofsted’s verdicts are only as reliable as flipping a coin.

Many rely on personal preference and value independent work over learning from a teacher, think-tank Policy Exchange claims.

Nearly three-quarters of teachers surveyed said they changed the way they worked during an inspection.

The think-tank called for routine lesson viewings to be scrapped and for better training for inspectors.

Its report has already inflamed tensions between Ofsted and the Government. Earlier this year, chief inspector Sir Michael Wilshaw said he was ‘spitting blood’ after learning that Policy Exchange and another right-leaning think-tank were preparing research questioning Ofsted’s effectiveness.

He said he suspected the Department for Education of a campaign to undermine him. Education Secretary Michael Gove replied that he backed Sir Michael ‘100 per cent’.

It later emerged Mr Gove had sacked the head of Ofsted. Baroness Sally Morgan – a Labour peer – claimed she was ousted in an attempt to politicise the watchdog, sparking further denials from Mr Gove and a fierce coalition row.

Today’s report says it found ‘widespread disillusionment with the inspection regime’, with teachers using Twitter and blogs to criticise lesson observations that may be only 14 minutes long.

She found that ‘very few’ highly praised lessons involved ‘teaching facts’, while ‘very often’ lessons were criticised for ‘the teacher talking too much’ or for ‘activities that involve factual recall’. The report notes that Sir Michael has warned inspectors on at least seven occasions not to favour one style of teaching.

However, it adds that it found a ‘consistent pattern in the preferred style of inspectors’ which often manifested itself ‘subconsciously’.

The report by Policy Exchange comes as some 189 out of 262 teachers polled said they changed their teaching for an Ofsted visit (library image)

One teacher said: ‘Everyone knows their current buzzword is “pace”. This means we have to assume children have the attention span of goldfish.’

Schools do ‘harmful’ amounts of preparation for Ofsted visits, which is a ‘complete waste of time’, the report says.

Some 189 out of 262 teachers polled said they changed their teaching for an Ofsted visit.

The report also says many of the 3,000 private inspectors lack training and experience.

Ofsted suggested last night that reforms were imminent, adding many of the report’s findings ‘chime with our own’.

Jonathan Simons, co-author, said the research suggests ‘you would be better off flipping a coin’ than relying on an inspector’s judgment of a lesson.

The think-tank is calling for shorter, more frequent inspections, followed by in-depth visits to struggling schools.

Ofsted schools chief Michael Cladingbowl said it is looking at reforms including giving parents more information. But he added: ‘In my view, parents will always expect inspectors to spend time in classrooms when they visit a school.’